I just found a reportedly complete 8.25" rear axle from a 1974 Dart on CL for cheap. I'd like to know how much/what kind of work will be required to put this thing under my 1964 Dart. I'd be replacing the stock 7-1/4" rear axle.

I've heard that this is a relatively straightforward bolt-in, but I just found this table, which says that the distance from axle centerline to rear U-joint is 1.6" larger on the 8.25" axle. That sure sounds like I'd need a different/modified driveshaft.

That table also says that the flange to flange width dropped by 1.8" after 1972. Not a huge deal, but I would definitely need spacers to continue using my current wheels.

The leaf spring mounting width is the same, thankfully. Does this mean that I can re-use my existing 1964 shock plates and U-bolts? Or am I in for a rude surprise there?

Finally, the seller has no idea what kind of gears this thing has in it, but it supposedly came out of a Dart with a 318. What is the most common gear ratio you'd expect in that setup? How difficult/expensive is it to change gear sets in the 8.25" axle? I'm currently running 2.76 gears behind my relatively stock 225, and it is truly horrible coming out of the corners in a road race. (My current rear axle was a replacement from a 1968 Valiant, hence the 2.76 instead of 2.93)

I would really like to use 3.55 or something similar. Maybe even higher, considering how much larger than stock my wheels/tires are. By my calculations, with 225/45-17 tires and a 3.73 rear gear, I could still just about hit 100 MPH at a 5000 RPM redline. Of course, if I continue driving the car around town between races, I'm sure I'll be looking for an OD transmission before too long...

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Yes. You will need to shorter your driveshaft by the difference in length between the 7 1/4 and 8 3/8 rear axle.
No, you won't. But you will need new wheels since the original 7 1/4 in your 64 Dart has the small 5x4 bolt pattern while the later 8 3/8 rear axle has the larger 5x4.5 bolt pattern. Your original wheels will not bolt to the larger rear axle.
Yes and no. You can re-use the leaf springs, but you will need at a minimum new u-bolts. Typically you also need the shock plates for the 8 3/8 rear axle, but if you have patience and can correctly use a measuring tape and a drill you can redrill the shock mounting plates from your original 7 1/4 axle. I did this on my brother's 74 Duster when I switched it from a 7 1/4 to a 8 3/8 rear axle.
An early 70s V-8 A-body would likely have rear gears in the 2.7-3.2 range. However, optional gear ratios were available and it is possible someone swapped in different gears in the 40 years since that axle was first built. Only way to know for sure is to pull the differential cover and count teeth on the ring and pinion. It isn't particularly difficult to swap gears. There are lots of web pages that talk about how to do it. The only caveat is if the 8 3/8 axle has 2.2 gears in it (The axle i put in my brother's 74 Duster has 2.2 gears). The 2.2 gears used a different carrier than all other gear ratios and the gear swap is a bit more involved.

I would 3.2 are my favorite gears for a daily drive light passenger car. 3.55 is better for more acceleration, but if you do lots of highway driving it an hurt MPGs a little bit. At the same time, if you do lots of stop and go driving or live in an area with lots of hills, 3.55 gears might actually save gas.

You kept saying 8-3/8", but I assume we're both talking about the same 10-bolt rear, right?

Yikes. If I'm going to be paying to shorten my driveshaft, I may as well see if the seller still has the shaft from the '74 Dart. Wikipedia says they're the same wheelbase. Actually, scratch that. My car has the trunnion joint at the transmission tailshaft. That would be useless, unless the guy just happens to have the car's transmission laying around as well. Or if I can find a newer replacement tailshaft... or a newer transmission... I guess I'll just shell out the $100 or so to cut the shaft down until I can afford to continue upgrading.

I'm already using higher-offset 5x4.5" wheels by way of billet aluminum adapters. The adapters are at least 1" thick, and if the new axle is almost another 1" shorter on either side, I'll probably lose all my clearance to the leafs on the inside.

If I need new U-bolts and have to redrill the shock plates, I assume that means that the axle housings will be bigger around. How much bigger are we talking? That level of fabrication shouldn't be a problem.

I guess if I do decide to buy this axle, I'll do a quick mark & spin when I pick it up, just to make sure it doesn't have 2.2 gears. Thanks, I wasn't aware of that possibility.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

The ring gear is actually 8 3/8 inches in diameter, but us lazy Mopar folks round down to 8 1/4 since the other two main passenger car rear axles (7 1/4 and 8 3/4) are in multiples of 1/4 inch. Factory literature refers to the 8 3/8 axle.

Shortening the driveshaft will be your easiest option.

I don't recall offhand the difference in the tube diameters between the 7 1/4 axle and the 8 3/8 axle. It is something small though, on the order of 1/2 inch or 3/4 inch. Someone more knowledgeable than I will be along shortly to answer this question.

Reed you gotta stop calling it the 8 3/8", only because the chrysler designation for the axle is 8 1/4"...Mopar did designate the 8 1/4" for trucks and vans as the 8 3/8", but also 8 3/8" is used to describe certain early 8 3/4" truck rear ends that used a smaller ring gear or were lighter duty and are not compatible with the c-clip 1969-current 8 1/4" rear.

I thought you put an 8 3/4" b-body rear under your brother's car anyway?

To answer quickly to help your situation:

The 1973-1976 A-body 8.25" rear will bolt under the other A-bodies no problem. Be aware that the rear has 10" or 11" drums depending on whether stock or someone wanted to have the "big drums". Your stock drive shaft will have to be shortened by 1.5-1.75". You can use stock mopar rims without problems on the rear, you may not lose much since the brake shoes are usually 3/4" wider than the 60's drum (2.5" vs. 1.75").

You will need shock plates for either an 8 1/4" or an 8 3/4" mopar rear...you can egg the holes in the 7 1/4" shock plates but it's not advisable. New wider U bolts are available over the counter at about $14 a pair (about $28 per car).

On axle ratios...when I pick up a rear end, the user if he's a real dealer, will have the rear cover removed so you can: 1) inspect the ring gear and pattern, 2) check the stamp on the gear for the ratio.


http://www.slantsix.org/forum/viewtopic ... light=4+56

That thread will help to see what is inside (be a bonus if a locker).

As it sits you will have to replace the carrier if the ratio is 2.56 or lower
(2.24,2.45,2.56), and you are wanting better gearing.

If it's a 2.71-4.56, you're good to go.

Stock ratios in 1974 could include 2.45 for 318-2v with A904 up to 3.23 open in HS 225 slant six 3 speed floor shift cars, 3.55 available in the 340/360 car up to 1976, where they changed the option to be 2.94 with 1 other optional ratio, later vehicles up to the 80's (especially the M/J bodies got the 2.24/2.56 ratios regularly).

Good Luck...in our location about $200 is OK for a low ratio axle (the 2.56-ish), $300 is OK for mid ranges, drag gearing with limited slip is in the $450-500 range as a bolt in.

-D.Idiot

Riiiight, I forgot the 10" drums are going to be wider than the 9" drums. That makes sense. The flange isn't the wheel mounting surface.

I'll see if the guy has the shock plates from the donor car. If not, I'll probably just egg the holes. Maybe weld on some reinforcement if it looks iffy.

When I spoke with the guy on the phone, he said the axle was complete with wheels and tires on it. Sounds like he pulled it off a car and it's been sitting. If I pick it up, I'll have to pull the cover off and check it out. The price is very good, so hopefully it's not rusted to hell.

Now that I'm looking at getting drive shaft work done, I'm strongly considering doing the u-joint conversion on the front. The ball & trunnion isn't making noise yet, but if I'm already taking everything apart, it might be a good time. Of course, I have to wonder if it's worth it to spend that much money on my current transmission, or just shell out the money to make it work as is and start looking for an OD upgrade.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

The 8 3/4 b-body rear axle went under my old Fifth Avenue that now lives with EricW down in Texas. OK, I'll butt out. I have only been reading mid 80s van factory service manuals lately, and they all talk about 8 3/8 rear axles only, (and danas, and spicers....)

You might be able to "cheat" and use a valiant drive shaft without cutting it down as the Dart is a little longer than the Valiant and the change in behind the front seat in the body/floor pan. Just an idea.

But getting rid of the BT joint would be nice as well.


-D.Idiot

Yeah the 8 3/8" has a little thicker ring gear as the change compared to the car thus the designation change for Van/Truck manuals . When you go to ask for parts though say Randy's Ring and Pinion-etc. it will have to be looked up as an 8 1/4" (Truck style gears are noted as being the 1976-1997 versions in the new diff catalogs, car versions would be found in the 1975-Down version), certain things interchange between Cars and Trucks, but not everything.

I apologize for being gruff, but your post just confused Frank and didn't give him a concrete direction to go or the correct designation that he could find his diff information on the web if needed.

-D.Idiot

You Road Race with adapters?

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1964 Dart GT

No worries. I fully acknowledge that I was giving bad information, and I do apologize to you and to Frank. Believe me, the last thing i want is to spread bad info. There are enough hacks yammering about things they don't know about on the internet, and I have absolutely no desire to be one of them. If I ever give wrong info, please call me on it since I have a tendency to shoot my mouth off before confirming what I am saying. Hence the disclaimer in my signature line.

Don't worry about it, Reed. You did give me some good info, and I managed to read through your funny numbers.

D.I., I did consider the different wheelbase question, but I don't think it would work. The next shortest wheelbase from my 111" is 108" (assuming that Wiki is correct). My driveshaft only needs to be shortened by 1.6". I think the only purely drop-in solution would be to grab the entire driveshaft out of a 111" wheelbase A-body that came with a 8.25" rear end, but as far as I know, none of those cars would also have the B&T joint. So I would also have to grab everything off the back of the transmission to convert my car to a front U-joint.

I'll definitely ask the guy selling the rear end if he has the transmission and/or driveshaft out of the donor car ('74 Dart), but I doubt it. I'll probably have to get my driveshaft shortened regardless of whether I change the front joint. Still, if the guy has the driveshaft but not the tranny, that would allow me to buy all the B&T conversion pieces myself and do the swap without a machine shop. Stop me if I've made a mistake in reasoning here...
Yeah... we've only been through one full race, but nothing's broken yet! -fingers crossed-

I should probably go ahead and check for signs of fatigue, now that I think about it. Aluminum doesn't have an endurance limit. This is another downside of doing this swap, though. Without the adapter spacing, I might have to buy new wheels again...

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Why not go with the FORD Explorer rearend (maybe even rear discs!), if you have to adapt your driveshaft anyway?

P.S.- I really like your Apollo paint scheme! (you have a Fan)

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1964 Dart GT

I had figured from your previous post that you might convert to the conventional slip yoke in which case there are longer slip yokes on the late 60's A-bodies that will take up that 1.5" slack with no problem (I have one from a 1967 Valiant A-904 and it's plenty long.

Another issue to address is the road racing, the 8.25" being a c-clip axle has been known to break the end of the old axle shaft when side loaded a lot (that's why the 8 3/4" was popular). Just letting you know.

Let us know what you find when you get there!

-D.idiot

Do you think the 8.25" axle would hold up better or worse in road racing than the 7.25"? I realize it's not as stout a unit as the 8.75", but this car will probably never be putting down enough power to justify spending money on a banjo axle. I might end up doing some kind of mild forced-induction setup some time next year if I get bored and crazy, but even then we're not talking anywhere in the neighborhood of 300+ HP. On the other hand, if the 8.25 isn't really a reliability improvement when it comes to road racing, then I shouldn't bother.

I would definitely like to change my gear ratio, so I figured it would be best to go up to a stouter (but still cheap) rear end rather than spending money on gears for the 7.25". If that's not a good line of thinking, then let me know. I'm pretty much flying by the seat of my pants here.

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing